Photoresponsive device including composition grading and recessed contacts for trapping minority carriers

Abstract

A mercury-cadmium-telluride (HgCdTe) photoresponsive layer (14) having the composition Hg 1-x Cd x Te is formed on a substrate (12) such that x increases from the surface (14a) of the photoresponsive layer (14) toward the substrate (12). This causes the bandgap in the photoresponsive layer (14) to increase from the surface (14a) toward the substrate (12), thereby urging minority carriers which are photogenerated in the photoresponsive layer (14) to move toward and be trapped at the surface (14a). Laterally spaced first and second ohmic contacts (16,18) are electrically connected to the photoresponsive layer (14) at a predetermined distance (z c ) below the surface (14a) such that the photogenerated minority carriers trapped at the surface (14a) are urged away from the contacts (16,18) by the increasing bandgap. An electrically floating photoresponsive layer (24) of opposite conductivity type may be formed between the substrate (12) and the photoresponsive layer (14) to form a Heterojunction Interface Trap.

We claim: 
     
       1. A photoresponsive device comprising: a substrate; and   a first photoresponsive layer formed on said substrate, said first photoresponsive layer having first and second opposing surfaces with said first surface further from said substrate than said second surface and having a bandgap which increases from said first surface toward the substrate and urges minority carriers which are photogenerated in said photoresponsive layer toward said first surface, said first photoresponsive layer having a first conductivity type; and   a second photoresponsive layer formed between said substrate and said first photoresponsive layer, said second photoresponsive layer having a conductivity type which is opposite to said first conductivity type.   
     
     
       2. A device as in claim 1, in which: said first photoresponsive layer has the composition A 1-x  B x  C, where A, B and C are first, second and third materials respectively; and   x increases from said first surface toward said substrate, thereby creating said increasing bandgap.   
     
     
       3. A device as in claim 2, in which the first material A is mercury, the second material B is cadmium and the third material C is tellurium. 
     
     
       4. A device as in claim 3, in which x increases at a rate of at least approximately 0.003/micrometer. 
     
     
       5. A device as in claim 3, in which said substrate comprises cadmium-zinc-telluride. 
     
     
       6. A device as in claim 2, in which the first material A is mercury, the second material B is zinc and the third material C is tellurium. 
     
     
       7. A device as in claim 1, further comprising first and second contacts which are laterally spaced from each other and electrically contact the first photoresponsive layer at a predetermined distance below said first surface such that said photogenerated minority carriers between said first surface and the first and second contacts are urged away from the first and second contacts toward said first surface by said increasing bandgap. 
     
     
       8. A device as in claim 7, in which said predetermined distance is selected such that said bandgap increases over said predetermined distance by approximately 5 kT, where k is Boltzmann's constant and T is the absolute temperature of said first photoresponsive layer.